The²⁹ Si NMR shielding tensor in low quartz

The spatial orientation of ²⁹Si chemical shielding tensor in low quartz relative to the crystallographic axes has been determined by a least-squares fit and matrix diagonalization of the chemical shift vs. angle data for a single crystal of amethyst. The values for the principal shielding axes of thetensor are σ₁₁ = −102.6, σ₂₂ = −107.0, σ₃₃ = −109.1ppm. To within 6°,σ₂₂ coincides with the a axis, and σ₁₁ with the line defined by the intersection of the plane of the two longer Si–O bonds with the plane perpendicular to the a axis passing through the Si site. A similar relation holds for σ₃₃ and the shorter Si–O distances. The data and orientations of the shielding tensor are consistent with previous studies that show the magnitude of the chemical shift to be inversely proportional to the Si–O bond distance. ²⁹Si nmr spectra taken from two orientations of a foliated quartzite show that the difference in the chemical shifts is consistent with the optical determination of the orientation of the quartz crystallites in the sample.